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Swiss security researchers have unearthed a flaw in wireless LAN systems that might be used by hackers to drastically increase their share of the available bandwidth at the expense of the other users. The issue should be of particular concern to hotspot operators, according to a team from the computer labs at the Ecole Polytechnique Federale de Lausanne (EPFL).

Appropriate standards (such as 802.11i) have been developed to ensure user security and privacy in hotspots, but this does nothing to prevent users altering the MAC protocol of a machine to increase his share of available bandwidth, according to the Swiss team.

They explain: "The new generation of wireless adapters allow easy modification of previously inaccessible MAC protocol parameters; for example, with a single line of code hackers can reduce the contention window size, realising a considerable redistribution of throughput shares among station competing for wireless bandwidth. Other cheating techniques include the modification of protocol timers, the misuse of collision-avoidance mechanisms such as the Net Allocation Vector, and selective scrambling of other users frames."

Professor Jean-Pierre Hubaux, leader of the three person team from EPFL who investigated the issue, said that although they had demonstrated these attacks in a lab environment they were yet to see reports about this kind of misdeeds in the real world yet. But that is no reason for complacency, he argued.

"Experience has shown that breaches are usually exploited, especially if this is easy to do (as it is the case here). With the increasing programmability of the devices, the risk will increase as well," Prof. Hubaux told El Reg.

"Considering that wireless access to hotspots is a charged service to a shared and scarce resource, it is easy to predict that numerous users will be tempted to cheat using the described techniques, thus discouraging honest users to make use of the service," the Swiss Boffins argue.

The Lab has also designed a (US patent pending) detection system, dubbed Domino, to spot bandwidth-stealing behaviour in wireless LANs. This technology is designed to help any Wi-Fi operator to protect its infrastructure against bandwidth-hogging hackers. EPFL hopes to license its technology to IT suppliers.

EPFL researchers will present their work at the Mobisys mobile system conference in Boston next week. ®